Catalytic hollow spheres

ABSTRACT

The improved, heterogeneous catalysts are in the form of gas-impervious, hollow, thin-walled spheres (10) suitably formed of a shell (12) of metal such as aluminum having a cavity (14) containing a gas at a pressure greater than atmospheric pressure. The wall material may be, itself, catalytic or the catalyst can be coated onto the sphere as a layer (16), suitably platinum or iron, which may be further coated with a layer (18) of activator or promoter. The density of the spheres (30) can be uniformly controlled to a preselected value within ±10 percent of the density of the fluid reactant such that the spheres either remain suspended or slowly fall or rise through the liquid reactant.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 83-568 (72 Stat435; 42 USC 2457).

This a division of application Ser. No. 602,901, filed Apr. 23, 1984,now U.S. Pat. No. 4,576,926.

BACKGROUND OF THE INVENTION

The present invention relates to heterogeneous catalysts and moreparticularly, this invention relates to hollow, gas-impervious,catalytic spheres having a preselected density for the controlled anduniform conversion of catalytically reactive fluid materials.

STATEMENT OF THE PRIOR ART

The relative merits of homogeneous and heterogeneous catalysts are wellknown. Homogeneous catalysts have better defined active sites, usuallyhave all of the metal available for catalysts, and offer steric andelectronic environments of the metal atom that can, at least inprinciple, be varied at will. The major disadvantage of homogeneouscatalysts is the need to separate them from reaction products withoutloss of their valuable metal content. This step can be both complex andexpensive. Other disadvantages are that these catalysts are relativelyeasily deactivated through aggregation or by poisonous by-products or atextreme temperatures. Also, corrosion of reactors by metal complexes iscommon.

Heterogeneous catalytic processes are of great industrial importance.Annually, 10,000 metric tons of ammonia are produced by directcombination of nitrogen and hydrogen gases at 400° C. and high pressureover iron catalysts promoted by several percent K₂ O and Al₂ O₃. Largevolumes of sulfuric acid and methanol are also produced by heterogeneouscatalysis. About 70 percent of all petrochemicals and refined petroleumproducts are produced by heterogeneous catalytic processes.Hydrogenation in presence of noble metals such as platinum or palladiumor transition metals such as nickel or cobalt can be used to convertcarbon monoxide to many different products such as ketones or alcohols,to convert olefins to alkanes, benzene to cyclohexane or nitro groups toamine groups. Transition metal catalysts also show activity for a widevariety of industrially important reactions such as isomerization,hydroformylation, carbonylation, etc. These catalysts can be used toconvert pyrolysis coal gases into synthetic fuels such as oxo alcohols.

Heterogeneous catalysts have been developed in which the homogeneouscatalyst is either impregnated onto or chemically bonded to a solidsupport. Reaction rate is also dependent on surface area, and manycatalysts are provided in finely divided form such as fine powders ofplatinum prepared by reduction of the oxide. Catalysts are also preparedby impregnating the active catalyst onto high area supports, forexample, platinum deposited onto alumina particles having surface areasof the order of 100 square meters per gram. Heterogeneous catalysts havebeen prepared by coating the catalyst onto a hollow, porous support.

Baer, et al. (U.S. Pat. No. 3,347,798) prepare hollow, catalytic beadshaving a diameter greater than 90 microns for a fluidized bed process.The beads are gas-permeable so that reactants can diffuse into the coreand react with the inner wall. The beads are formed by spraying ahydrogel such as silicic acid or alumina containing a vaporizableexpanding agent through a nozzle into a tower and impinging the streamwith a gas heated to 300°-700° C. Pilch, et al. (U.S. Pat. No.3,538,018) disclose an improvement over Baer, et al., in which compactcatalyst particles are added to the hollow spheres to form a mixturehaving a controlled density. Ao (U.S. Pat. No. 3,798,176) manufacturescontrolled-density, catalyst pellets having a vacant or a dense center.Ao forms a vacant center pellet; a thin polymeric shell is coated with acarrier and catalyst particles. During calcination, the core burns awayand the particles sinter and consolidate into a gas-permeable shell. Inthe dense center pellet, the core is made by pelletizing a ceramic.Martin (U.S. Pat. No. 3,978,269) forms porous pellets for automotiveexhaust reactors by coating liquid droplets with a powder mixture ofceramic and binder and then firing to form a porous, breatheable, hollowpellet. Watson, et al. (U.S. Pat. No. 4,039,480) also form an automotivecatalyst by coating a dry core with a dispersion of ceramic and firingto form a product having a bulk density below 50 lbs/ft³. Barnes, Jr.(U.S. Pat. No. 4,292,206) incorporates tiny, hollow glass spheres in thealumina powder mix as a light-weight filler to reduce weight of theresultant catalyst beads.

These porous catalytic particles are not uniformly dispersed in thefluid reaction media. They require separation of the reactant andproduct from the catalyst. The ceramic or polymer supported catalystparticles tend to crack, corrode or decay, which clogs the catalyst bedand requires shutting the reactor down to replace the bed and incurs theexpense of replacing the catalyst. Irregularly-shaped catalysts are notan optimum shape for catalytic reaction kinetics. The metal catalyst areheavy and would sink to the bottom of the reactor unless the reactionmedia is stirred or the catalyst circulated through it.

STATEMENT OF THE INVENTION

Improved heterogeneous catalysts are provided in accordance with thepresent invention. The catalysts of the invention have a uniform,controlled density that can be preselected such that the spheresdisperse uniformly throughout a fluid reaction medium or rise or fallthrough the medium at a preselected rate. The catalyst particles of theinvention are very strong and physically tough and will not crack, chipor abrade. The particles can be used and reused for numerous runs beforerequiring any regeneration or reprocessing. The catalyst of theinvention provides a large surface area for optimum contact of reagentand catalyst while assuring unobstructed flow of reactants through thebed of catalyst.

The catalyst of the invention automatically distributes through thereagent or can flow through the body of reagent without requiringshaking, stirring or pumping. The catalyst particles are readilyprepared in large volume and uniform shape at low cost by the process ofthe invention.

The improved catalyst provided by the present invention is in the formof gas-impervious, hollow, thin-walled spheres. The wall material mayitself be catalytic or the catalyst can take the form of a coating ontothe wall material. Additional layers of activators or promoters can becoated onto the sphere or the promoters or activators can be mixed intothe layer of catalyst. The density of the spheres can be accuratelycontrolled by controlling the internal gas pressure and/or wallthickness of the spheres to form a uniform batch of spheres in which theweight of the spheres varies less than ±5 percent.

Spheres that are from 0.5 to 10 percent by weight lighter than thereaction media will slowly rise at a controlled rate through thereaction media. Spheres that have substantially the same density (±0.01to 0.5 percent by weight) as the reaction media will remain uniformlydispersed therein and spheres that are from 0.5 to 10 percent heavierthan the reaction media will slowly fall through the media. The spherescan be produced over a fairly large range in diameter, such as from 0.20to 5.0 millimeters and still provide sufficient surface area for thecatalytic reaction to proceed at an economic rate.

The spherical catalysts are readily dispersed with a minimal amount ofenergy and can reject exothermic heat to the surrounding reaction media.The catalyst of the invention is easy to handle and readily separatesfrom the reaction media for cleaning, reprocessing, regeneration orrecirculation. There is no problem with packed beds or with fluid flowsince the catalyst spheres maintain a uniform dispersion with separationbetween adjacent spheres. The catalysts of the invention provide optimumutilization of expensive catalyst materials since the catalyst materialsare provided on the surface. The inner, inert core of the particles isfilled with inert gas. The catalysts of the invention are applicable forall prior heterogeneous reactions such as hydrogenation, polymerizationor oligomerization, isomerization, etc.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a catalyst sphere according to theinvention;

FIG. 2 is a schematic view of a batch reactor containing a uniformdispersion of catalyst spheres;

FIG. 3 is a schematic view of a fluidized bed reactor containing a bedof catalyst spheres;

FIG. 4 is a schematic view of a reactor with a falling column ofcatalyst spheres;

FIG. 5 is a schematic view of a reactor with a rising column of catalystspheres; and

FIG. 6 is a schematic view of a continuous flow reactor with continuouscirculation of catalyst and continuous introduction of reactant andremoval of reaction product.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the catalyst of the invention is in the form ofa hollow sphere 10 having a gas-impervious shell 12 formed of a metal.The hollow, interior cavity 14 contains a gas such as air underpressure. The presence of the gas and the thickness and weight of theshell and any promoter or initiator layers thereon are selected toprovide a predetermined density in relation to the density of the fluidreaction media. The shell may be formed of a catalytically active metalsuch as platinum or iron or may be formed of another metal such asaluminum on which is coated a layer 16 of catalyst, e.g. platinum oriron. The catalyst layer 16 may be from 0.1 μm to 0.1 mm in thickness.The catalyst layer 16 may contain from 1 to 25 percent by weight of apromoter initiator such as an alkali metal oxide, or a thin layer 18from 0.1 μm to 0.1 mm in thickness of the promoter, or the initiator maybe coated onto the surface of the catalyst layer. The layers 16, 18 maybe formed by deposition of the compounds from the vapor or liquid phasesuch as by thermal decomposition of metal carbonyls and the like.

Gas-filled, spherical, metal shells, that are dimensionally precise,smooth and of high strength can be produced by a method based on thehydrodynamic instability of an annular jet of molten metal as disclosedin U.S. Pat. No. 4,344,787, the disclosure of which is expresslyincorporated herein by reference. The basis of the method rests upon thephenomenon of instability and breakup of a jet flow of liquid as itissues into a gaseous medium at rest. In the embodiment employed herein,a coaxial flow of fill gas is provided at the core of a circular jet bymeans of a thin-wall tube. When the axial velocities of the jet liquidand of the central gas are adjusted to fall within certain ranges, thejet exhibits an instability which generates large-amplitude axisymmetricoscillations. These culminate in a rapid pinchoff of the jet and in theformation of liquid shells which can be described as thick-wall bubbles.A remarkable feature of the instability is that it is more powerful byfar than the familiar Rayleigh instability of a nonhollow jet.

The oscillation growth is so rapid that the nonlinear motion regime isattained within three or four jet diameters, and pinchoff ensuresquickly. The motion is highly deterministic; although the action occursspontaneously and without external stimulus, a frequency stability andcorresponding uniformity in shell mass exceeding one part in 10³ isreadily attained. As each shell in turn parts from its neighbors, itundergoes a ringing oscillation which has the beneficial effect ofpromoting a centering of captured gas.

The dimensions of shells produced by nozzles of this type may be variedover wide limits. In accordance with the physical process of shellformation, the diameter of the product will be approximately twice thatof the jet orifice, whereas the relative wall thickness is not so simplydetermined. That quantity is not only a function of the aspect ratio ofthe annular passageway, but is simultaneously a function of the volumeflow rate of the fill gas. An increase in the gas flow rate at fixedliquid rate results in an increase in the bubble formation frequency andin a concommitant decrease in wall thickness.

Tin and aluminum shells ranging in diameter from 750-2000 μm and wallthicknesses of about 25 μm have been formed in quantity. Here, the metaljet issued into ambient air. Examination of specimens was made by meansof scanning electron micrography (SEM). For shells at the upper sizelimit, it was found that the specimens were spherical to within aboutone percent except near two diametrically opposing points from which thejet pinchoff had occurred.

Metallic shells have been produced by the jet instability method atrates up to a few thousand per second. The shells exhibit excellentuniformity in size, good sphericity over most of the surface and fairconcentricity. The spheres have excellent surface quality and hightensile strength. The shells are then further coated with catalyst andpromoter layers. As a specific example of practice, aluminum spheresabout 2 mm in diameter, with a wall thickness of about 25 μm and aninternal air pressure of about 100 psi could be coated with a 0.1 μmthick layer of platinum by reduction of precursor oxide powder.Palladium or nickel could also be coated onto the alluminum shell bythis technique. These catalysts could be utilized for hydrogenationreactions. When the aluminum shell is coated with iron, the spheres canbe utilized to produce ammonia by combining N₂ and H₂ gases at 400° C.and several hundred atmospheres of pressure. Further coating the ironlayer with a layer of Na₂ O or K₂ O results in a synthesis gasconversion catalyst which converts CO and H₂ gases into product gascontaining CH₄, C₂ H₆, C₃ H₈, C₉ H₁₀, other alkanes, olefins, alcohols,aldehydes and acids.

The ability to adjust the buoyancy of the uniformly-sized spheres makepossible reaction processes in which the body of reactant is stationaryand the catalyst moves through the reactant at a controlled rate, or isuniformly suspended therein. A process utilizing catalytic, hollowspheres 30 having a density differing from the liquid media 32 by ±0.1to 0.5 weight percent is illustrated in FIG. 2. As the catalystparticles 30 are fed from the hopper 34 into the reactor 36, they willdeploy to form a uniform suspension 37 within the body 36 of liquidreactant. This reactor can be operated as a batch reactor in whichreactants are introduced through inlet 38 and are removed at the end ofthe run through outlet 40. The catalyst can be separated by means of ascreen or filter 42 and recycled to the hopper 34.

Continuous flow processes can be operated by flowing reactant through abed of catalyst restrained between porous barriers such as screens. Thecatalyst spheres may tend to pack against the upstream screen unlessthey are allowed to expand as in a fluidized bed reactor as shown inFIG. 3. The catalyst particles 30 are placed in the reactor 31downstream of perforated plate 42. The gaseous or liquid fluid reactantis introduced through the inlet 44 placed upstream of the plate. Theparticles 30 expand by the action of the flowing stream to form afluidized bed 46. A further screen or perforated plate 48 may be placedtowards the top of the reactor to prevent any catalyst particles frombeing carried out the outlet 50 with the reaction products.

A batch reactor with an autogeneously moving catalyst suspension isillustrated in FIG. 4. In this process, the catalyst particles 52 are ofuniform size and density and have a density preselected to a value from0.5 to 10 percent heavier than that of the liquid reactant media 54within the reactor 56 so that the transit time of the particles withinthe reactor provides a desired degree of conversion of the reactants. Asthe catalyst spheres 52 deploy into the reaction media 54 from thehopper 58, they will form a uniform suspension which slowly falls to thebottom of the vessel at a controlled rate. Reactants can beintermittently or continuously fed to the reactor from the inlet 60 andreaction product can be intermittently or continuously removed throughoutlet 62 containing a liquid-solid separator 64 to remove catalystparticles 52 for recycle to the hopper 58.

FIG. 5 illustrates a batch or continuous process utilizing catalystparticles 70 which are lighter than the reaction media by 0.5 to 10percent by weight and have a density preselected to provide a desiredrate of travel of the particles through the column 72 of reactant. Thespherical catalyst particles 70 are fed from a supply vessel 74 into thebottom of the reactor 76 and slowly rise as a suspension through thecolumn 72 of reactant. The catalyst particles can be removed by askimmer 78 and recycled through line 80 to the supply vessel 74. Thereaction product can be recovered through an outlet 82 or by overflowintermittently or continuously. Similarly, reactant material can beintroduced through the inlet 84 continuously or intermittently.

In the embodiment shown in FIG. 6, the catalyst suspension and reactantcolumn move continuously under counter-current flow conditions. Thisprocess also illustrates use of two different catalysts for a two-stageprocess. The reactor 80 includes two circulation loops 83, 85. In thefirst loop 83, a first reactant, R¹, is fed into the inlet 86 from thesupply tank 88 and pump 90 and forms a rising column 92 of reactant.First stage product can be removed through outlet 94 and recycledthrough line 96 to the inlet side of the pump 90 when the three-wayvalve 98 is turned toward the recycle line 100.

First stage catalyst 91 is fed into the inlet 102 of loop 85 from hopper104, rises through the column 92 of reactant and is removed throughoutlet 106. The first stage catalyst is recycled through line 108 untilthe reaction is complete. Valve 110 is turned toward line 112 and allthe first stage catalyst is returned to the hopper 104. Valve 114 isopened to feed second stage catalyst 116 into the loop 85 and reactioncolumn 92. Additional reactant, R², may now be fed from tank 116 intoinlet 86 by opening valve 118.

The three-way valve 98 is turned toward recycle. The second stagecatalyst is cycled through the column 92 until the reaction is complete.The three-way valve is then turned toward vessel 120 and the secondstage reaction product is recovered.

It is to be understood that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications, andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A reaction medium comprising a fluid reactant containing auniform suspension of uniformly-sized, hollow, gas-impervious catalystspheres in the form of a gas-impervious shell containing a gas at apressure greater than atmospheric and having a layer of catalyst on thesurface thereof, said spheres having a preselected density not varyingmore than ±10 percent of the density of the fluid reactant.
 2. Areaction medium according to claim 1 in which the spheres have a densitywithin ±0.5 percent of the density of the fluid reactant and form auniform stationary suspension therein.
 3. A reaction medium according toclaim 1 in which the spheres have a density less than the density of thefluid reactant by 0.5 to 10 percent and form a suspension that slowlyrises therethrough.
 4. A reaction medium according to claim 1 in whichthe spheres have a density greater than the density of the fluidreactant by 0.5 to 1.0 percent and form a suspension that slowly fallstherethough.
 5. In a method of catalytically converting a fluid reactantinto a product, the improvement comprising the steps of:forming auniform suspension of uniformly sized, hollow, gas-impervious catalyticspheres within a body of the fluid reactant, said spheres being in theform of a gas-impervious shell containing a gas at a pressure aboveatmospheric, having a layer of catalyst on the surface thereof andhaving a preselected density within ±10 percent of the density of thefluid reactant; catalytically reacting the reactant by means of thecatalyst layer to form a reaction product; and separating the catalystspheres from the reaction product.
 6. A method according to claim 5 inwhich the spheres have a density within ±0.5 percent of the density ofthe fluid reactant and form a uniform stationary suspension therein. 7.A method according to claim 5 in which the spheres have a density lessthan the density of the fluid reactant by 0.5 to 10 percent and form asuspension that slowly rises therethrough.
 8. A method according toclaim 5 in which the spheres have a density greater than the density ofthe fluid reactant by 0.5 to 10 percent and form a suspension that slowyfalls therethrough.
 9. A method according to claim 5 in which thediameter of the spheres is from 0.20 mm to 5.0 mm.
 10. A methodaccording to claim 9 in which the spheres are formed of metal.
 11. Amethod according to claim 5 in which the catalyst layer includes atransition metal or noble metal catalyst and the catalyst furtherincludes an activator or promoter.
 12. A method according to claim 11 inwhich the activator or promoter is present as a separate layer.